Process for the manufacture of surface-treated metallic sheets or the like,with superior coating and anti-corrosive performance



0% 6, 1970 SH JICHIRO TAKIGUCHI ETAI- 35 PROCESS FOR THE MANUFACTURE OF SURFACE-TREATED METALLIC SHEETS OR THE LIKE, WITH SUPERIOR COATING AND ANTI-CORROSIVE PERFORMANCE Filed may 23, 1966 United States Patent US. Cl. 204-56 9 Claims ABSTRACT OF THE DISCLOSURE A process for treating the surface of a metallic sheet stock comprising mixing a chromic acid reducing agent with an aqueous emulsion of a modified synthetic rubber, and adding thereto a mixture of chromic acid and phosphoric acid, applying a film of the treating liquor to the surface of the stock, and curing the coated product at elevated temperatures. The modified polymers used in making the emulsion are either acrylonitrile-butadiene or vinylidene chloride having polarizing radicals such as carboxylic, epoxy or amides. A preferred composition for the aqueous emulsion solution is:

(1) an emulsion of acrylonitrile-butadiene or vinyl chloride having polarizing radicals,

(2) dispersant liquid containing a minute amount of Formalin and polyoxyethylene octyl phenol ether, (3) a reducing agent such as millet-jelly or sugar.

This invention relates to improvements in and relating to a process for the surface-treatment of metallic sheets, bands, plates and the like plan stocks, preferably those of steel, galvanized steel, alloy steel, light metal and the like.

Surface-treatment of metallic sheets has been mainly effected by utilization of synthetic resin emulsions, chromate solutions, phosphate solutions or the like.

Considerable defects and drawbacks have been encountered with the use of these surface-treated metallic sheets in that inferior bonding of surface-finish-coatings, such as paint and the like protective coatings occurs. If these are applied to the surface of the sheet stock poor anti-corrosive performance of these coatings per se and low grade machinability of the coated sheets occurs.

As shown in the foregoing the principles of the present invention are well applicable generally to metallic sheets and the like. The following description of the invention will be directed mainly to galvanized steel sheets or plain steel sheets as the representatives of such sheets, although the invention should not be limited thereto.

It is an object of this invention to provide an improved process for the manufacture of surface-treated steel sheets capable of providing superior bonding performance to ornamental, protective or finish-coatings such as paint coatings.

It is another object of the invention to provide a process for the surface-treatment of the above kind, the treated sheets exhibiting superior anti-corrosion performance.

It is still another object of the invention to provide a process of the above nature for producing treated sheet having superior machinability.

A still further object is to provide a process for the manufacture of surface-treated steel sheets adapted for use in place of tinned can sheets and provided with surface coatings highly suitable for canning purposes, and,

in addition, capable of providing superior performance as sub-layers for ornamental, protective and other useful finish-coatings.

For realizing aforementioned several objects, it is now proposed, according to this invention, to adopt a unique process which comprises, in combination, selecting a member from the group of, styrene-butadiene rubber, acrylonitrile-butadiene, vinylidene chloride and the like; introducing into said member a polarizing radical such as or the like; preparing an aqueous emulsion of said selected and polarized member; mixing said emulsion with chromic acid and phosphoric acid; preparing an evenly distributed dispersion therefrom; applying said dispersion to a metallic sheet, subjecting the thus treated sheet to a reducing treatment for converting almost all of the hexavalent chromium to trivalent chromium. In the course of the preparation of said dispersion, a reducing agent for chromic acid is preferably added, if the reducing process is not electrolytically carried into effect.

As a preferred and representative way of introducing polarizing radical into the main constituent of the treating liquor, taking as an example COOH, specific organic acid such as maleic acid, acrylic acid or the like, which contains both vinyl radical and carboxyl radical, is added to the monomer of the synthetic rubber and subjected to copolymerization.

The aforementioned polarizing radicals are given only by way of example, and thus the invention is not limited to these specific radicals. It will be however Well understood from the description to be set forth that the inventive technique is sharply contrasted to known techniques employing non-polarized starting materials for the desired purpose. In the process proposed by the present invention, two or more polarizing radicals can be simultaneously utilized.

Although the precise reaction mechanism of the introduced polarizing radicals in the formation of organic and/or inorganic complex compounds in the protective surface coatings on the steel sheets processed by the present invention is at present not wholly clarified, it Will become clear from Table I, to be set forth, that according to the proposed technique a remarkable bond performance between the formed protecting surface coatings and the sheet stock can be realized.

The accompanying drawings, FIG. 1 is an electro microphotograph taken from a surface-treated steel sheet according to Example 1.

FIGS. 2 and 3 are views similar to FIG. 1, illustrating, for comparison, conventional products procured from market, more specifically those subjected to chromic acid and pigmented chromic acid treatment, respectively.

FIG. 4 is a similar view of a conventional phosphatetreated, zinc coated steel sheet.

FIG. 5 is a similar view to FIG. 4, taken from a conventional chromate-treated, zinc coated steel sheet.

FIG. 6 is a similar view to FIG. 4, taken from a conventional electrolytically chromate-treated steel sheet.

EXAMPLE 1 In the following, a treating liquor for general use is first described. Tinned can sheets can be treated by diluting the general-use liquor with 2-6 parts of water per part of liquor. The treating liquor is a mixture of liquid A and liquid B. Liquid A further comprises liquid I which is a synthetic rubber emulson, liquid II which is a dispersant liquid, and liquid III whch is a reducer solution. Liquid B comprises further liquid IV and liquid V. This preparation can be seen from Table I, to be given hereinafter. Percentages are given by Weight, if not otherwise specified.

TABLE I.COl\lPOSIIION OF TREATING LIQUOR Component Liquid Synthetic Rubber Liquid (Liquid (A)):

Synthetic rubber emulsion (Liquid 1) Ingredients Weight, percent {Styrenc-butadienc rubber, carboxylatcd. 40-60 1 60 80 IW-ater with dispersant 40-60 l Polyoxyethylenc octyl phenol ether 10-30 Dispersant liquid (Liquid II) Water 69. 9-88. 10-20 -40 FQllllfi lllLn 0. 1

Reducing liquid (Liquid H1) 10-20 Total 100.0

Cln'omate (Liquid (13)):

Chromate liquid (Liquid IV) Zn0 70-90 Water Phosphoric acid solution (Liquid V) Total sum Preparation of liquid A Carboxylated styrene-butadiene rubber, 40-60%, and water mixed with a small amount of conventional dispersant such as polyoxyethylene phenol ether (dispersing agent), 40-60%, were throughly mixed together while agitating, to for an evenly distributed disper- III are mixed together in a ratio of 60-80%, 10-20% and 10-20%, respectively while strongly agitating to obtain the liquid A.

Preparation of liquid B 20-40% of CrO 10-30% of ZnO (buffer or intensify ing agent) and of water are throughly mixed while strongly agitating, to obtain liquid IV. Next, -90%, preferably 85.0%, industrial H PO and 10-20%, preferably 15%, of water are thoroughly mixed to form said liquid V. Then, said two liquids IV and V are mixed in 5 Preparation of treating liquor Next, liquid A and liquid B are mixed in a ratio of 20-40%, preferably 30%, and 10-30%, preferably 20%, respectively, with water 50%, with strong agitation, to obtain a homogeneous dispersion ready for use.

Treatment of strip stock Then a metallic strip stock, preferably bare steel strip or zinc-coated steel strip, is treated with the aforementioned liquor according to the conventional dipping process, so that the pick-up quantity become 3-7 g./m. and then cured for a short time, preferably 2-3 minutes at 260-330 C., as measured on the cured strip coil. In this way, rigidly attached and stable protecting coatings are formed on the both surfaces of the stock.

The thus surface-treated coil is then cut to desired size and subjected to a finish or ornamental coating process with paint or other protecting agent. In comparison with conventional metallic sheets coated with synthetic resin paints, the aforementioned surface-treated sheets are tested and the results are set forth in following Table II. Table III shows the evaluation standards adopted in Table I.

TABLE II Paint Bond Test B 0 Salt Epoxy Ericksen water Mela- Tliermo- Mela- Melamodified Alkyd test (extruspray mine setting mine mine melamine derivasion Wear test alkyd aeryl alkyd alkyd alkyd tive Tested material Kind of sheet stock 3 mm.) test (24 h.) resin resin resin resin resin paint Inventive product Cold rolled steel sheet 8 10 6 10 10 10 10 10 8 D0 Zinc-coated steel sheet" 6 6 6 10 10 10 10 10 8 Chromic acid treated commer- Cold rolled steel sheet 4 1 l 1 1 1 1 10 1 cial product. Chomate treated commercial do 4 1 1 1 4 1 1 10 1 product.

Do Zinc-galvanized steel sheet 1 1 1 1 6 1 1 10 1 TABLE III Wear test, as Ericksen test, measured upon Salt water spray Bond test, as damaged area, fn'ctional treattest, as measmeasured by Evaluation percent, when ments as set ured by rusted still bonded standard extruded 3 nun. forth area, percent area.

1 Less than once.

The aforementioned treating liquor is then diluted with four volumes of water and coated on a can sheet. Then, the coated stock is baked or cured at 330 C. for 2 minutes. Comparison test results are shown in the following Table IV. It will be seen from these results that the treated metallic sheets produced according to this invention are remarkably superior.

and protective purpose, will be considerably lowered. Even this temperature is considerably higher than conventionally employed value when a similar coating is formed from chromic acid and any selected resin material.

The protecting layer formed by the process according to this invention comprises a highly stable complex com- TABLE IV Commercially Chromic procured Commercial Zinc-galvanized Performance Test acid electrolytically chromium late Inventive treatchromate treated plated Item Conditions products ment steel sheet steel sheet N o. 25 No. 50

Machinability test:

Picked-up quantity Gn/m. 055-060 0.22 0. 16 0.20 2. 79 6. 59 Erickson test Ball dia. 20 mm., extrusion 3 mm. 8 1 4t 6 8 8 Machinability test 3 mm., 180 bending 10 1 6 6 1 6 Impact test Shocking pin, dia. 127 mm., load 6 1 1 4 4 4 300 g., height 500 mm. Coating performance test:

Repellent nature to paint Alkyd series resin for can use 1O 10 10 10 10 10 Scratch test ..do 10 10 10 10 8 8 Phenolic paint for can use 10 6 6 4 Epoxyphenol series paint for can 10 6 8 6 Bond test use.

Rosin-modified alkyd reminous 10 10 10 10 10 10 Alkyd resinous 10 6 6 4 Printing performance test:

Epoxy resinous paint for can use. 10 G 6 4 paint for can use.

Degree of eye hole {Rosin modified alkyd resinous Alkyd resinous llgpoxy resnfou; paint for can use. osin-mo i re al yd resinous-.. Bond test paint for can use.

Alkyd resinous paint for can use. Antiposting performance test:

Wetting test 50C, 05-98% 5 l1 Salt water spray test 5% NaCl, 350, 3 l1 Room residing test Room temperature; 50 days Anti-chemical test:

Citric acid dip test 10%; room temperature; 5 l1 Caustic soda dip test 50%; room temperature; April Antisolvent test:

Toluol dip test Room temperature, April Ligroin dip tost do Trichlorocthylene dip test do Ethyl alcohol do Acenton dip test .do Anti-oil test:

Mineral oil test 38 C; March Vegetable oil test .do Anti-cleaning agent test:

Dip test in Wonderful K. Room temperature; April Dip test Newbease do Dip test "Mypet do 10 1O 1O 10 1O 10 10 10 10 10 10 10 10 10 10 10 8 10 10 10 10 10 8 8 10 10 10 10 10 10 In the above Example 1, polyoxyethylene octyl phenol ether is used as the dispersing agent. Other dispersants can also be utilized with similar results when they do not react with the employed constituting chemicals to gel products. As reducing agent, sucrose, glycol, glycerine and/or the like, can be equally employed in place of millet-jelly shown in Example 1, by way of example. The dip process may also be replaced by sprayor other co-nventionel coating processes.

The curing temperature can be elevated to as high as 700 C., when measured at the temperature of the heating medium. In this case, the curing period can be substantially shortened, for instance, to several seconds. By adopting these processing conditions, the working efiiciency can be highly improved.

When occasion demands, a liquid composition having the same constituent percentage, yet without the addition of any reducing agent, is applied to a metallic stock under the same operating conditions as above mentioned, then subjected to an electrolytic reducing step and finally cured as before. The products show similar anticorrosive performance and machinability.

When the treating temperature is raised above the aforementioned specific highest temperature, part of the resinous component would crack. Thus the coating would be considerably inferior in comparison with the expected values. On the other hand, if the coil temperature as measured on the treated coil is lowered below 260 C., the bond performance to a paint when it is applied to the products according to this invention for ornamental pound, although its structural formula can not be specifically determined, of synthetic rubber, chromic acid and phosphoric acid, wherein the orginal hexavalent chromium component has been substantially or more specifically as high as 96100%, converted to trivalent chromium by a reducing action. The reason why such a result could be obtained is supposedly based substantially upon the introduction of polarizing radicals into one reaction components or more specifically the synthetic rubber component, on one hand, and upon the high temperature curing as above specified, on the other hand.

There have been known treating compositions comprising chromic acid and reducing agent, occasionally in combination with resinous component, adapted for the purpose of treating a metallic surface, and these layers are coated thereon, which process is characterized by a reducing process for keeping intentionally left remained part of the included, hexavalent chromium component for specific purpose to be realized. On the contrary, for carrying out the process wherein, as already mentioned, the resin component introduced thereinto polarizing radicals are used in combination with chromic acid (preferably added with reducing agent for the latter) and phosphoric acid, the reducing action should preferably be carried into effect so that almost none of the hexavalent chromium component remains, if high performance of bond of paints to be applied later, and highly improved machina'bility are desired.

If the content of the synthetic rubber component is lowered beyond the specific values as shown in Table I, the bond performance will be considerably injured. When, on the contrary, the rubber content is increased beyond the specified values, the anti-corrosive performance of the formed layer will become inferior, for the manufacture of marketable products.

If the chromium content should be lowered beyond the specified value in Table I, the anti-corrosive performance becomes inferior, and when the component is increased above the highest specified value therein, the bonding per formance of finished paint coatings may be injured.

The metallic surface treated with the aforementioned treating liquor has a White-green tone. Organic and inorganic pigments or the like conventional coloring agents can be added to the coating liquor, if ornamental elfects are wanted. These agents should also be used in the form of finely divided particles so as not to injure the desired bonding performance of the coated layer according to the invention. The treated sheet products can be used for various and numerous purposes without applying ornamental color paint layers. Although this invention can be well applied to tinned or zinc-coated steel sheets, it is a advantageous that the sheet products, as treated according to this invention, can well be produced at a considerably more economical base than the tinned sheets and be used in place of thereof. Thus, by adopting the present invention, highly economical can sheets may be produced in place of tinned iron sheets.

EXAMPLE 2 In place of carboxylic radicals, epoxy, acid amide and/ or CONHCH OH radicals are introduced into styrenebutadiene rubber, and further treatments are carried into effect under similar operating conditions as set forth in Example 1 with similar results.

EXAMPLE 3 40% of acrylonitrile-butadiene introduced with carboxylic radicals and 60% of water added with a minor quantity of conventional dispersant, as employed broadly in the rubber industry, are thoroughly mixed together while agitating, so as to produce 60 parts of synthetic rubber emulsion.

Next, a dispersant solution is prepared as in the case of Example 1, from of polyoxyethylene octyl phenol ether, 89.9% of water and 0.1% of Formalin (37%).

Further, a reducing liquid is prepared from millet-jelly, 50%, and Water, 50%, as before.

Then, these three separately prepared liquids are intimately mixed together to prepare a type of liquid A.

Next, CrO 40%, Zn, 10% and water, 50% are intimately mixed together, While industrial H PO 85.5% and water, are mixed to a solution. Both of these liquids are then mixed together at a ratio of 7:3 to form a liquid B.

Then, 200 parts of liquid A, 300 parts of liquid B and 500 parts of water are intimately mixed to prepare a treating liquor as mentioned hereinbefore, and applied to a hot steel strip at 320 C., as measured on the strip coil directly treated therewith, for 2.5 minutes for curing, the coating weight being 5 g./m. so as to form thin films on 'both of the surfaces of the sheet stock. The results are similar of those set forth in Example 1. The percentage of chromium converted to trivalent chromium was as high as 100%. Therefore, no fear was encountered from solving-out of the chromium while in use.

EXAMPLE 4 60 parts of vinylidene chloride, introduced therein with epoxy radicals as polarizing radicals, and 40 parts of water, added with a small amount of conventional dispersant as in the preceding examples, are thoroughly mixed together to form a synthetic rubber emulsion, while a dispersant solution comprising 30% of a conventional dispersant, as was employed in Example 1, 69.9% of water and Formalin (37%) 0.1%, is prepared as before. Further, sugar 40% and water 60% are intimately agitated to form a reducing liquid. Then, these three kinds of liquids are combined together at a ratio of 70:15:15, so as to produce a kind of liquid A, in the aforementioned meaning.

Next, liquid B is prepared as was shown in aforementioned Example 1, the percentages being capable of varing within limits listed in aforementioned Table I.

Then, 400 parts of liquid A, 100 parts of liquid B and 500 parts of water are intimately mixed together to form the treating liquor in the aforementioned meaning. The operating conditions are as before and the resulting performances are, similarly, very superior as in the case of preceding examples.

FIG. 1 shows an electromicroscopic photograph of the product prepared in Example 1 X5000). FIGS. 26 are similar views to FIG. 1, taken from comparative conventional products. From comparison of these photographs, it may be easily ascertained that the inventive productive product (with no pigment added thereto) represents a highly even and fine film structure. There are shown in FIG. 1, a number of small spots which are believed at present to comprise fine particles. However, there is no evidence for discriminating them from recessed spots.

It is to be noted that, for carrying out the invention acrylonitrilebutadiene rubber, vinylidene chloride and the like used as the starting material should have a mutual solubility to chromic acid.

Although several preferred examples of the present invention have been described hereinbefore, various and numerous modifications and alterations can be easily thought out upon reading through the present specification by those skilled in the art. These modifications and alterations should be included within spirit and scope of the present invention so far as they are within the framework of the appended claims.

What we claim is:

1. A process for the surface treatment of metallic sheet stock, comprising preparing an aqueous mixture comprising:

(a) from 20 to 40 Weight percent of a mixture of:

(1) from 60 to weight percent of an aqueous emulsion of a polymer comprising monomer precursors selected from the group consisting of acrylonitrile-butadiene, and vinylidene chloride, said polymers containing members selected from the group consisting of -COOH, =C-C=, CONH2, and CONHCH2OH (2) from 10 to 20 weight percent of a dispersant liquid, and

(3) from 10 to 20 percent by Weight of an aqueous solution of a chromic acid reducing agent, and

(b) from 10 to 20 weight percent of a liquid contain- (1) from 70 to weight percent of an aqueous solution containing from 20 to 40 weight percent of a hexavalent chromium compound and 10 to 30 percent by weight of zinc oxide, and

(2) from 10 to 30 weight percent of an 80 to 90 weight percent phosphoric acid solution;

coating said stock with said aqueous mixture, and curing the coated stock at a temperature of from 260 to 700 C., so that substantially all of the hexavalent chromium is converted to trivalent chromium.

2. The process of claim 1, wherein said polymer is carboxylated poly- (styrene-butadiene) 3. The process of claim 1, wherein said polymer precursor is acrylonitrile.

4. The process of claim 1, wherein said polymer precursor is vinylidiene chloride.

5. The process of claim 1, wherein said dispersant liquid comprises an aqueous solution containing 0.1 Weight percent of 37% Formalin and to weight percent polyoxyethylene octyl phenol ether.

6. The process of claim 1, wherein said reducing agent is an aqeuous solution of millet-jelly or sugar.

7. The process of claim 1, wherein said coating weighs from 3 to 7 grams per square meter of stock.

8. The process of claim 1, wherein both sides of said stock are coated.

9. A process for the surface treatment of metallic sheet stock, comprising preparing an aqueous mixture comprising:

(a) from 20 to weight percent of a mixture of:

(1) from to 80 weight percent of an aqueous emulsion of a polymer comprising monomer precursors selected from the group consisting of acrylonitrile-butadiene, and vinylidene chloride, said polymers containing members sel cted from the group consisting of (2) from 10 to 20 weight percent of a dispersant liquid, and (b) from 10 to 20 weight percent of a liquid containing:

(1) from to 90 weight percent of an aqueous solution containing from 20 to 40 weight percent of a hexavalent chromium compound and 10 to 30 percent by Weight of zinc oxide, and (2) from 10 to 30 weight percent of an to weight percent phosphoric acid solution; coating said stock with said aqueous mixture, subjecting said aqueous mixture to electrolytic reduction, and curing the coated stock at a temperature of from 260 to 700 0., so that substantially all of the hexavalent chromium is converted to trivalent chromium.

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